My email notification doesn't use the custom ringtone

In the sounds and haptics menu in the device settings, I've set up a custom ringtone as the default notification for a new incoming mail. And in my mail client (I've tried both Spark and Yahoo), I've set the notification settings to use the device default. However, whenever I get an email, it uses some third noise altogether?? Please help, I find this notification sound insufferable and I don't understand why setting up a custom notification sound is so complicated!